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Surface & Coatings Technology 190 (2005) 75–82

Preparation of Fe–Cr–P–Co amorphous alloys by electrodeposition

C.A.C. Souzaa,*, J.E. Mayb, A.T. Machadoa, A.L.R. Tacharda, E.D. Bidoiac

aDepartamento de Ciencias e Tecnologia dos Materiais, Escola Politecnica-DCTM, Universidade Federal da Bahia, Rua Aristides Novis, no. 2,

Salvador/BA, Federacao, CEP 40210-630 Salvador, BA, BrazilbDepartamento de engenharia de Materiais, UFSCar, Brazil

c Instituto de Biociencias, UNESP–Rio Claro, Brazil

Received 9 July 2003; accepted 8 April 2004

Available online 9 June 2004

Abstract

The effects of bath composition and electroplating conditions on structure, morphology, and composition of amorphous Fe–Cr–P–Co

deposits on AISI 1020 steel substrate, priorly plated with a thin Cu deposit, were investigated. The increase of charge density activates the

inclusion of Cr in the deposit. However, above specific values of the charge density, which depend on the deposition current density, the Cr

content in the deposit decreases. This Cr content decreasing is probably due to the significant hydrogen evolution with the increasing of

deposition current and charge density. The effect of charge density on the content of Fe and Co is not clear. However, there is a tendency of

increasing of Fe content and decreasing of Co content with the raising of current density. The Co is more easily deposited than the P, and its

presence results in a more intense inhibition effect on the Cr deposition than the inhibition effect caused by P presence. Scanning electron

microscope (SEM) analysis showed that Co increasing in the Fe–Cr–P–Co alloys analyzed does not promote the susceptibility to

microcracks, which led to a good quality deposit. The passive film of the Fe–Cr–P–Co alloy shows a high ability formation and high

protective capacity, and the results obtained by current density of corrosion, jcor, show that the deposit with addition of Co, Fe31Cr11P28Co30,

presents a higher corrosion resistance than the deposit with addition of Ni, Fe54Cr21P20Ni5.

D 2004 Published by Elsevier B.V.

Keywords: Electrodeposition; Amorphous alloys; Cobalt

1. Introduction

The corrosion resistance of amorphous Fe-based alloys

containing Cr and the elements P, Ni, and Co has been

investigated [1]. It has been reported that the corrosion

resistance of such alloys is improved by the presence of

these elements. In view of those results, the use of Fe-based

alloys in corrosion protection is very promising.

The higher corrosion resistance of amorphous alloys

compared with crystalline alloys with the same composition

is well known. However, this effect is related to the presence

of strongly passivating elements such as Cr. Amorphous

alloys without strongly passivating elements such as amor-

phous Fe–B and Co–B alloys have a lower corrosion

resistance than their crystalline counterparts [2]. This higher

corrosion resistance of amorphous alloys has been attributed

to two effects: the introduction of chemical heterogeneity in

0257-8972/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.surfcoat.2004.04.070

* Corresponding author. Tel./fax: +55-71-332-1254.

E-mail address: caldassouza@hotmail.com (C.A.C. Souza).

crystalline alloys, which prevents the formation of a uniform

passive film [3,4], and the active dissolution of amorphous

materials, which promotes the accumulation of passivating

species at the alloy–solution interface prior to formation of

the passive film and results to rapid formation of a highly

protective passive film such as hydrated chromium oxy-

hydroxide [1,5].

The addition of P promotes the formation of the fully

amorphous structure of Fe-based alloys and maintains high

corrosion resistance [6]. Moreover, addition of P in Fe-

based alloys containing Cr supports the formation of the

chromium-enriched passive film and promotes the increase

of corrosion resistance [7].

There aremany reports about the corrosion behavior of Fe-

based alloys obtained by ‘‘melt-spinning’’ process. However,

this method makes possible just the manufacture of alloys in

ribbon form, which limits its application. Another method

commonly used to obtain Fe-based amorphous alloys is

electrodeposition. This method appears to be highly compet-

itive compared to other methods. Two of its more important

Table 1

The concentrations and functions of reagents present in the plating bath

Reagent Concentration (M) Function

CrCl3�6H2O (chromium chloride) 0.38 Source of Cr

FeCl2 (ferrous chloride) 0.16 Source of Fe

NaH2PO2 (sodium hypophosphite) Source of P

Cl2Co�6H2O (cobalt chloride) Source of Co

Na3C6H5O7�2H2O (sodium citrate) 0.32 Buffer agent

NaBr (sodium bromide) 0.15 Antioxidizing

H3BO3 (boric acid) 0.5 pH reductor

NH4Cl (ammonium chloride) 0.9 Complex agent

HCOOH (acid formic) 0.9 Source of C

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–8276

advantages are the easy preparation of samples in different

shapes and geometries and the possibility of changing the

alloy composition within a broad range, changing only the

deposition parameters. In addition, electrodeposition is an

inexpensive method of materials preparation.

Concerning amorphous alloys containing Fe and Cr

obtained by electrodeposition, there are in literature studies

about Fe–Cr [8], Fe–Cr–P [9], Fe–Cr–Ni [10], and Fe–

Cr–P–Ni [11,12] alloys. The Cr is added in amorphous

electrodeposited alloys due to its high capacity of corrosion

inhibition. However, this element can also affect the struc-

ture of the alloy. This effect was reported [8] in Fe–Cr alloy

films electrodeposited, and it was shown that the Cr pro-

motes the formation of a full amorphous structure at 22.9

at.%. The structural effect on the corrosion resistance of

amorphous electrodeposited alloys was also analyzed by

Sziraki et al. [10] in Fe–Cr–Ni alloy (25 at.% Cr, 25 at.%

Ni, and 50 at.% Fe). The amorphous, microcrystalline, and

crystalline structures were investigated. The results show

that the amorphous electrodeposited alloys had both the

highest ability to spontaneous passivity and the highest

corrosion resistance when compared to the microcrystalline

and crystalline samples. The authors attributed the good

results of the amorphous alloys to the lower number of

active sites in the amorphous structure.

Electrodeposition of amorphous Fe–Cr–P and Fe–Cr–

P–Ni alloys was achieved by Kang and Lalvani [9,11]. The

Fe–Cr–P alloys were deposited on steel substrate, priorly

plated with a thin Cu deposit. The Fe–Cr–P–Ni alloys were

deposited on Au substrate. The authors reported that to

obtain a good quality deposit, a cell divided by a cation-

selective membrane must be used. At the anode, oxidation of

Cr3 + to Cr6 + occurs, and consequently, the deposits obtained

from Cr6 + are thin and inhomogeneous. Thus, the presence

of a cation-selective membrane between the cathode and

anode hinders the deposition from these Cr6 + ions, improv-

ing the quality of the deposits. It was also reported [9] that

addition of formic acid, as a source of carbon, enhances the

appearance of deposits.

Kang and Lalvani [11] reported that the Cr content in the

Fe–Cr–P–Ni deposits rises with the increase of both

current and charge density deposition. However, these

studies were restricted to a limited range of current and

charge densities. Our prior work [12] analyzed the electro-

deposition of Fe–Cr–P–Ni alloys at a large range of

current densities deposition (200, 300, 400, and 500 mA/

cm2) and charge densities deposition (50, 100, 150, 200, and

300 C/cm2) on steel substrate, priorly plated with a thin Cu

deposit. The results show that the increase of charge density

causes an initial increase of the Cr content in the deposit.

However, above a specific value of the charge density,

which depends on the current density deposition, the Cr

content decreases. This is attributed to the significant

hydrogen evolution. The results also show that the increase

of Ni, Cr, or charge deposition promotes the susceptibility to

microcracks. The deposit composition and deposition

parameters were optimized and an amorphous Fe–Cr–P–

Ni deposit with high Cr content (Fe54Cr21P28Ni5) and with a

minimal presence of microcracks was obtained at 500 mA/

cm2 and 150 C/cm2.

Investigations [1] with the amorphous alloys obtained by

‘‘melt-spinning’’ reported that the addition of Ni is more

effective in improving of corrosion resistance than the

addition of Co. However, the Ni presence in the Fe–Cr–

P–Ni electrodeposited alloys is limited to small contents

[11,12], probably due to the effect of anomalous codeposi-

tion caused by Fe presence.

In view of the advantages of the electrodeposition

process and the protective characteristics of Fe–Cr–P

amorphous alloys, the use of these electrodeposited alloys

is very promising in the protection of a substrate with low

corrosion resistance, such as steel. An example of applica-

tion of these electrodeposited alloys is the protection against

corrosion of Fe–Si alloys with soft magnetic properties

which present low useful life in aggressive environment.

However, the addition of Cr is detrimental to magnetic soft

properties [8,13]. Soft magnetic materials require a good

magnetic flux density; therefore, the Co presence is inter-

esting in the electrodeposited alloy, which enhances these

properties [14].

The electrodeposition process of Fe–Co- and Fe–Co–P-

based alloys has been investigated [15–17] due to the

interesting soft magnetic properties of these alloys. Howev-

er, in our view, this the first time that electrodeposition of

amorphous samples of Fe–Cr–Co–P is reported.

The purpose of this work is therefore twofold: first, it

presents new Fe–Cr–Co–P electrodeposited amorphous

alloys, and second, it shows the effects of bath composition,

deposition charge, and current density on the deposits’

characteristics.

2. Experimental procedure

Electrodeposition of alloys was carried out from a

chloride solution containing Cr, Fe, P, and Co sources and

additives. The additive concentrations were equal to that

used in the electrodeposition of FeCrPNi [9,12] alloys. The

composition, concentrations, and functions of each one of

these reagents are listed in Table 1. Different concentrations

Table 2

Composition of Fe–Cr–P–Co alloys obtained from different conditions of

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–82 77

of NaH2PO2 and CoCl2�6H2O were used. The pH was

adjusted to a feasible range, around 1.8, with addition of

HCl.

Electrodeposition was carried out at room temperature

(25 jC) under galvanostatic conditions. Alloys were depos-ited on AISI 1020 steel disc substrate (surface-exposed area:

0.264 cm2), priorly plated with a thin Cu deposit in order to

increase the deposit adherence to substrate. Platinum foil

was used as a counter electrode and potentials were mea-

sured against a saturated calomel electrode (SCE). A cation-

selective Nafion (N-324) membrane was used to separate

the catholyte from the anolyte.

The electrodeposited composition was determined by

energy dispersive X-ray spectroscope (EDS), performed in

a Carl Zeiss (model DSM 940 A) scanning electron micro-

scope (SEM) equipped with an energy dispersive X-ray

analyzer. The structure of deposits was investigated by X-

ray diffraction (XRD) using a Carl Zeiss URD6 automatic

difractometer set at 40 kV and 20 mA with filtered CuKa

radiation.

Deposit adherence was evaluated by the adhesive ribbon

method in accordance with ASTM 3359-83 standard. De-

posit thickness and the current efficiency of deposition was

evaluated from mass gain and density of the deposit [18].

The corrosion resistance of the samples was analyzed by

polarization potentiodynamic curves and current density of

corrosion, jcor, at room temperature (25 jC). These measure-

ments were carried out in an aerated acid solution (H2SO4

0.1 M) with a Potentiostat/Galvanostat AUT30.FRA2. v

Basic Autolab. For each deposit analyzed, current density of

corrosion and potential of corrosion of three samples were

measured. Results that are shown correspond to the media

of these measurements. The auxiliary electrode was a

platinum foil and a saturated calomel electrode (SCE) was

used as a reference. The polarization potentiodynamic

curves of substrate in the absence and presence of deposits

were carried out at a 5-mV/s� 1 scan rate.

current density, j, and charge density, qd

Sample j qd Composition (at.%) Deposit

(mA/cm� 2) (C/cm� 2)Fe Cr P Co

adherent

A 200 50 12 5 41 42 No

B 200 100 17 6 37 40 No

C 200 150 16 6 37 41 No

D 200 200 20 7 32 41 No

E 300 50 23 5 32 40 No

F 300 100 22 6 30 42 No

G 300 150 21 6 30 43 No

H 300 200 27 7 28 38 Yes

I 400 50 30 6 35 29 No

J 400 100 26 7 33 34 Yes

L 400 150 28 8 28 36 Yes

M 400 200 31 6 29 34 Yes

N 400 300 32 6 28 34 No

O 500 50 32 7 36 25 No

P 500 100 32 8 25 35 Yes

Q 500 150 31 11 28 30 Yes

R 500 200 31 9 24 36 No

S 500 300 30 8 28 34 No

3. Results and discussion

3.1. Effect of deposition parameters

Initially, the effects of deposition parameters (current

density, j, and charge density, qd) on the composition and

adherence of Fe–Cr–P–Co films were investigated. The

deposits were obtained from bath deposition listed in Table

1, containing 0.23 M NaH2PO2 and 0.17 M CoCl2�6H2O.

These concentrations are like P and Ni source concentra-

tions used in deposition of FeCrPNi alloys [9,12].

The deposit was considered adherent when it was not

reported by visual observation in the presence of remnant

deposit on the adhesive ribbon after it has been withdrawn

from deposit. However, when it was reported in the presence

of remnant deposit, it was considered not adherent. Adher-

ent deposits were obtained at 400 and 500 mA/cm2, and at

charge densities of 100 and 150 C/cm2. The results in Table

2 show that at low current and charge density, the conditions

of deposition are not enough to promote the formation of

adherent deposits. However, above a specific value of the

charge density, which depends on deposition current densi-

ty, the adherence to substrate decreases. This behavior is

probably related with the high evolution of hydrogen at

higher charge density, which can increase the tension of the

deposit [12] and decrease its adherence.

The results in Table 2 also show that the composition of

the deposit is affected by deposition parameters ( j and

qd).There is an increasing tendency of Cr content in the

deposits with the increasing of j and qd. This observation is

in agreement with the results obtained by Kang and Lalvani

[9,11] for electrodeposits of Fe–Cr-based alloys and is

expected because chromium deposition occurred at the

most electronegative potential compared with all the ele-

ments presented in the bath. However, it was observed that

at high current density (400 and 500 mA/cm2) above a

specific value of the charge density (150 C/cm2), the Cr

content decreases with the charge density. This decreasing

of Cr content is probably due to the significant hydrogen

evolution with the increasing of current deposition and

charge density. Because of the highly hydrogen evolution,

the pH at the metal–solution interface increases and a Cr

hydration occurs, consequently decreasing the Cr deposi-

tion efficiency.

In previous work [12], Fe–Cr–P–Ni electrodeposits

were obtained from j and qd from different deposition

conditions and from bath deposition similar to those listed

in Table 1 (0.17 M NiCl2 and 0.23 M NaH2PO2 as sources

of Ni and P, respectively). The results in Table 2 show that

the Co relative content is higher in Fe–Cr–P–Co alloys

compared with the Ni content in Fe–Cr–P–Ni alloys

Fig. 1. Composition of Fe–Cr–P–Co alloys obtained from different P

source (NaH2PO2) concentrations at 500 mA/cm2 and 50 C/cm2. (x), Fe;(n), Cr; (E), P; and (.), Co.

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–8278

although the concentrations in the bath of Co and Ni sources

are the same. This behavior shows that Co is easily

deposited than Ni. The behavior reported in Table 2 for

the Cr content in deposits of Fe–Cr–P–Co alloys is similar

to that observed for Fe–Cr–P–Ni [12] electrodeposited

alloys. However, the Cr content is higher in Fe–Cr–P–Ni

alloys, which is connected easily to Co deposition compared

with Ni deposition.

On the other hand, Table 2 also shows that there is an

increasing tendency of Fe deposition and a decreasing

tendency of Co deposition with the increase of current

density. However, the effect of charge density on the content

of those elements is not clear.

Table 3 shows the deposit thickness and the current

efficiency of deposition of Fe–Cr–P–Co alloys obtained

from different conditions of deposition reported in Table 2.

These results indicated that the effect of current density, j,

on the deposit thickness and the current efficiency is

coherent with the effect of this deposition parameter on

the deposit with addition of Cr. The results show that there

is an increasing tendency of the deposit thickness and

current efficiency, as the Cr content, with the increasing of

current density. However, at higher current densities (400

and 500 mA/cm2) and higher charge densities (200 C/cm2

and 300 cm2), this effect is not observable and there was no

discernible influence of the charge passed on the current

efficiency. These results can be related with the high

hydrogen evolution at higher charge and current densities,

which can inhibit the deposition. In relation to the effect of

charge density, qd, on the current efficiency and thickness of

deposit, the results show that the increase of qd from 50 to

100 C/cm2 increases the current efficiency and thickness of

the deposit. However, the increase of qd from 100 C/cm2

does not result in continual increase of current efficiency,

and at high current density (400 and 500 mA/cm2), the

increase of qd results in decrease of current efficiency. These

results probably are consequences of raising of hydrogen

evolution with the increase of charge density and current

density of deposition.

The effect of P source concentration on the composition

of Fe–Cr–P–Co deposits is shown in Figs. 1 and 2. The

Table 3

The deposit thickness and the current efficiency of Fe–Cr–P–Co alloys

obtained from different conditions of current density, j, and charge

density, qd

Sample Thickness

(Am)

Efficiency

(%)

Sample Thickness

(Am)

Efficiency

(%)

A 2 11.6 J 16 46.3

B 10 29.3 L 20 38.7

C 12 23.0 M 21 30.4

D 15 22.2 N 21 20.1

E 3 14.5 O 6 29.5

F 14 34.8 P 18 52.1

G 17 25.0 Q 22 45.6

H 18 26.0 R 21 30.5

I 5 20.7 S 21 20.3

deposits were obtained at 500 mA/cm2, from bath deposi-

tion listed in Table 1 (containing 0.17 M CoCl2�6H2O and

different NaH2PO2 concentrations).

The results shown in Figs. 1 and 2 indicate that at both

deposition charge densities, the Cr content in deposits

increases in the absence of P source in bath deposition.

However, it is not observed that there is a continuous

increase of Cr content with the decrease of NaH2PO2

concentration. Moreover, in the deposits obtained at 150 C/

cm2, it is observed that there is a decrease of Cr content.

These results can be attributed to a significant increase of Co

content in the deposit due to the decrease of P content,

resulting in inhibition of Cr content increase. Concerning the

Fe presence in the deposits, the results show that in the

depositions at 50 C/cm2, the effect of P source concentration

on the presence of this element in the deposits is not clear.

However, it is observed that in the deposition at 150 C/cm2,

there is an increasing of Fe content in the deposits with the

decreasing of the P source concentration.

Fig. 3 shows the effect of Co source concentration on the

composition of Fe–Cr–P–Co alloy. The deposits were

obtained at 500 mA/cm2 and 150 C/cm2 from bath deposi-

tion listed in Table 1 (containing 0.23 M NaH2PO2 and

different CoCl2�6H2O concentrations). All deposits obtained

were considered adherent to substrate by the adhesive ribbon

method. It can be observed from Fig. 3 that there is a

tendency of Cr and Fe content in deposits to increase and

P content to decrease with the decrease of CoCl2�6H2O

Fig. 2. Composition of Fe–Cr–P–Co alloys obtained from different P

source (NaH2PO2) concentrations at 500 mA/cm2 and 150 C/cm2. (x), Fe;(n), Cr; (E), P; and (.), Co.

Fig. 3. Composition of Fe–Cr–P–Co alloys obtained from different Co

source (Cl2Co.6H2O) concentrations at 500 mA/cm2 and 150 C/cm2. (x),Fe; (n), Cr; (E), P; and (.), Co.

Fig. 4. XRD patterns for the alloys obtained at 500 mA/cm2 and 150 C/cm2.

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–82 79

concentration. These results and those reported in Figs. 1 and

2 indicate that the Co is easily deposited than the P.

Moreover, the Co presence results in a more intensive

inhibition effect on the Cr deposition in comparison with

the effect caused by P presence.

The current efficiency of deposition and the thickness of

deposits vary between 21–23 Am and 45–48%, respective-

ly, as reported in Table 4. These results reinforced that the

effect of alloy composition on current efficiency and deposit

thickness of those elements is not clear.

3.2. Characterization of structure and morphology of

electrodeposits

The X-ray diffraction pattern for all deposits obtained

showed only a broad peak (100 peak correspondent to

Fe-a), which is typical from an amorphous structure [19].

Fig. 4 shows the typical XRD pattern, which corresponds

to Fe31Cr11P28Co30, Fe38Cr18Co44, Fe33Cr12P25Co30, and

Fe39Cr15P23Co23 electrodeposits obtained at 500 mA/cm2 at

charge densities of 150 C/cm2. The objective in obtaining

the amorphous structure in this work is related with the

higher corrosion resistance of this structure. The presence of

this structure leads to rapid enrichment of Cr ions at the

alloy–solution interface and to a rapid formation of hydrated

oxyhydroxide films with a high protective quality [1].

The presence of amorphous structure in the alloys con-

taining P is expected because it is well known that the

addition of high P content promotes the amorphous structure

formation. It was reported that [7] the Fe–8Cr–P alloy with

Table 4

The deposit thickness and the current efficiency deposition of Fe–Cr–P–

Co alloys obtained from various Co source concentrations at 500 mA/cm2

and 150 C/cm2

Deposit Thickness (Am) Current efficiency (%)

Fe56Cr27P17 22 47.3

Fe53Cr26P18Co3 21 45.1

Fe48Cr16P25Co11 22 45.8

Fe48Cr16P20Co16 21 44.0

Fe39Cr15P23Co23 23 48.3

Fe33Cr12P25Co30 22 46.0

Fe31Cr11P28Co30 22 45.6

20 at.% of phosphorus forms a completely amorphous

structure. This result was confirmed by transmission elec-

tron microscopy.

It has been reported [19] that at least 15 wt.% of P- to Fe-

based electrodeposits is necessary to present an amorphous

structure. However, the X-ray diffraction patterns of the

Fe38Cr18Co44 electrodeposits, without P, show a typical

behavior of an amorphous structure, a broad peak. However,

it is not possible to affirm that the structure of this alloy is

completely amorphous. Kang and Lalvani [11] reported by

X-ray diffraction a behavior typical of amorphous structure

to FeCrNi alloy. This alloy also was obtained from bath

deposition containing acid formic as source of carbon and

the amorphous structure formation was attributed to the

presence of this element. However, in our work and in the

Kang and Lalvani work, only X-ray diffraction was used.

Therefore, it is necessary to use another method such as

transmission electron microscopy to verify the presence of a

completely amorphous structure. This investigation is envis-

aged in future work.

The effect of composition on the morphology of electro-

deposits is shown in Fig. 5. This figure shows SEM micro-

graphs of electrodeposits obtained at 150 C/cm2 and 500

mA/cm2 from bath deposition listed in Table 1 (containing

0.23 MNaH2PO2 and different CoCl2�6H2O concentrations).

The microcracks’ presence in SEMmicrographs is related

to the electrodeposits quality. Then, a significant presence of

Fig. 5. SEM photomicrographs of the surface of electrodeposits obtained at 150 C/cm2 and 500 mA/cm2: (a) Fe48Cr16P25Co11 and (b) Fe31Cr11P28Co30.

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–8280

microcracks is related to the deposit brittleness, which leads

to deterioration of mechanical and corrosion resistance

properties. The presence of significant microcracks can lead

to direct contact between a substrate and the corrosive

environment, which results in galvanic cell formation with

the substrate behaving as anode and the deposit as cathode.

This behavior results in intensive corrosion of a substrate

region in direct contact with the environment. The presence

of a corroded region located in the substrate, which behaves

as amplifier place of tension, can lead to deterioration of

mechanical resistance. In another work [20] about the

FeCrNiP electrodeposited alloys, we have reported that the

presence of significant microcracks decreases strongly the

corrosion resistance in relation to deposits with a minimum

presence of microcracks.

Kang and Lalvani [11] have investigated the morphology

of Fe–Cr–P–Ni electrodeposits by SEM and reported the

absence of microcracks for 1000� magnification. In this

present study, the electrodeposits of Fe48Cr16P25Co11 and

Fe31Cr11P28Co30 alloys were investigated by SEM, and it

also reported the absence of microcracks on these deposits to

1000� magnification. Therefore, in order to analyze the

effect of composition on the microcracks of deposits, 2000�magnification was used.

Fig. 5a and b corresponds to 2000� magnification and

shows that the increase of Co content in the deposits does

not promote the increase of microcracks. These results

suggest that the presence of Co does not lead to increase

of tension of electrodeposits investigated.

The microcracks’ presence depends on the deposition

conditions and the composition of electrodeposits. The

higher Cr content promotes the tension of deposits and

therefore the microcracks’ formation [12]. The presence of

microcracks is also related to high hydrogen evolution at

Fig. 6. Curves of potentiodynamic polarization of substrate in absence of

deposit (—) and in presence of Fe39Cr15P23Co23 (. . .. . .), Fe31Cr11P28Co30(- - - -), and Fe54Cr21P20Ni5 (– � – � – ) deposits. These curves were

obtained in 0.1 M H2SO4 solution with a scanning rate of 5 mV/s� 1.

Table 5

Current density of corrosion, jcor, and potential of corrosion, Ecor , ob-

tained in 0.1 M H2SO4 solution, of alloys deposited at 500 mA/cm2 and

150 C/cm2

Deposit Icor (A) Ecor (V vs. SCE)

Fe54Cr21P20Ni5 1.42� 10� 3 � 0.398

Fe31Cr11P28Co30 4.63� 10� 4 � 0.432

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–82 81

higher charge deposition [12]. However, in this paper, it was

reported that the Cr content in the deposits analyzed

(Fe48Cr16P25Co11, and Fe31Cr11P28Co30) and the conditions

of deposition (500 mA/cm2 and 150 C/cm2) are not enough

to promote the significant presence of microcracks. It is also

reported that the P content in the deposits does not cause the

presence of significant microcracks.

The SEM micrographs show that it is possible to

obtain electrodeposits of alloys (Fe48Cr16P25Co11 and

Fe31Cr11P28Co30) adherent to substrate with higher Cr

and Co content and a minimum presence of microcracks.

3.3. Corrosion resistance of electrodeposits

Potentiodynamic polarization was carried out in 0.1 M

H2SO4 solution to investigate the effect of Fe–Cr–P–Co

deposits on the corrosion resistance of substrate and achieve

the comparative experiment data between the corrosion

resistance of Fe–Cr–P–Ni coating and the Fe–Cr–P–Co

coating. The comparative experiment between these coat-

ings was also carried out from obtainment of current den-

sity of corrosion, jcor. The deposits analyzed were the

Fe54Cr21P20Ni5 coating and Fe31Cr11P28Co30 coating. These

coatings were obtained at same deposition conditions (500

mA/cm2 and 150 C/cm2) and from plating bath containing

the same concentration of Ni (0.17 M NiCl2) and Co sources

(0.17 M CoCl2�6H2O). The other components of the plating

bath are the same. Although the concentrations of Ni source

and Co source have been equal in the bath, the Co content is

much higher than the Ni content in the deposit. This

behavior indicates that the Co is easily deposited than Ni.

The Fe54Cr21P20Ni5 coating exhibiting an X-ray diffrac-

tion pattern [12] showed only a broad peak (100 peak

correspondent to Fe-a) which is similar to the X-ray

diffraction pattern of Fe31Cr11P28Co30 coating and is typical

from an amorphous structure.

Fig. 6 shows the potentiodynamic polarization curves of

substrate (AISI 1020 steel-plated with a thin Cu deposit) in

the absence of the deposit and in the presence of Fe39Cr15P23Co23, Fe31Cr11P28Co30, and Fe54Cr21P20Ni5 depos-

its. These results show that in the absence of the deposit,

the current density after the corrosion potential (� 0.400

V vs. SCE) increases continually, indicating the absence

of a passive film. However, for the samples recovered

with deposits immediately after the corrosion potential

(� 0.432 V vs. SCE, � 0.390 V vs. SCE, and � 0.398 V

vs. SCE for Fe31Cr11P28Co30, Fe39Cr15P23Co23, and

Fe54Cr21P20Ni5, respectively), only smaller changes of

the current density can be observed, indicating the pres-

ence of a passive region, which is extended up to

potential at around 1.5 V vs. SCE. This behavior indicates

that the deposits’ presence produces a protective inhibition

effect.

The immediate formation of the passive film after the

corrosion potential and the low density current at the passive

region reported for the deposits investigated is a typical

behavior of amorphous alloys with high corrosion resistance

[1]. This behavior is related to the higher reactivity of

amorphous structure, which leads to a rapid enrichment of

element former of passive film at the alloy–solution inter-

face and a rapid formation of passive film with a higher

protective quality. However, it was not possible to report

with clearness the difference among the potentiodynamic

polarization curves of Fe54Cr21P20Ni5 coating and FeCrPCo

coatings.

The current densities of corrosion, jcor, obtained for

Fe54Cr21P20Ni5 coating and Fe31Cr11P28Co30 coating are

shown in Table 5. The results show that the deposit content

Co presents a lower jcor and consequently a higher corrosion

resistance than the deposit content Ni, although the Cr

content in the deposit content Ni is higher. It is known [1]

that the Ni presence in amorphous Fe-based alloy results in

a higher corrosion resistance than the Co presence. Howev-

er, the higher corrosion resistance of Fe31Cr11P28Co30coating can be related with the higher content of Co in

deposit in relation to Ni. In addition, there is a possibility of

a synergetic effect between Cr and Co. However, this

possibility must be investigated.

The Fe–Cr–Co–P electrodeposit is an interesting alloy

not only due to its corrosion resistance but also for its

magnetic properties due to the Co presence, which enhances

the magnetization saturation flux density [14]. Therefore, the

Fe–Cr–Co–P electrodeposited alloy is an interesting meth-

od of protection against corrosion of magnetic soft substrate

such as Fe–Si alloy. Moreover, the results obtained in this

work indicate that the use of Fe–Cr–Co–P coating in the

protection against corrosion of magnetic soft substrate such

as Fe–Si alloy is more interesting in comparison with the

Fe–Cr–Ni–P alloys because it can allow the decrease of Cr

C.A.C. Souza et al. / Surface & Coatings Technology 190 (2005) 75–8282

content in the alloy, which is dangerous to magnetic soft

properties, without having to decrease the corrosion resis-

tance of deposit. Therefore, a study about the effect of these

alloys on magnetic properties of substrate such as Fe–Si

alloy used as magnetic soft material is interesting and is

envisaged in future work.

4. Conclusion

Amorphous Fe–Cr–P–Co electrodeposits with high Co

and Cr contents were obtained. These deposits exhibit

characteristics of passive film with a minimal presence of

microcracks and high corrosion resistance. The increase of

Co content in the deposits of alloys analyzed does not

promote the susceptibility to microcracks. The results

obtained by current density of corrosion, jcor, show that

the deposit with addition of Co, Fe31Cr11P28Co30, presents a

higher corrosion resistance than the deposit with addition of

Ni, Fe54Cr21P20Ni5, although the Cr content in the deposit

with Ni is higher.

The increase of charge density activates the inclusion of

Cr in the deposit. However, above a specific value of the

charge density, which depends on the deposition current

density, the Cr content in the deposit decreases. There is a

tendency of the Fe content to increase and Co content to

decrease with the increase of current density. However, the

effect of charge density on the content of these elements is

not clear. The presence of Co and P inhibits the deposition

of Cr in Fe–Cr–P–Co alloys. Co is easily deposited than P,

and its presence results in a more intensive inhibition effect

on the Cr deposition compared with the effect caused by P

presence.

The results show that there is a tendency for the deposit

thickness and current efficiency of deposition to increase

with the increase of current density. However, to higher

current and charge density, this effect is not observable. The

increase of charge density does not result in continual

increase of current efficiency and thickness deposit, and to

high current density (400 and 500 mA/cm2), the increase of

charge density results in decrease of current efficiency.

Acknowledgements

The authors are grateful for the financial support from the

Brazilian foundations FAPESP and FAPESB.

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